Without limiting the scope of the invention, its background is described in connection with Influenza virus.
Influenza A and B viruses cause a highly contagious respiratory disease in humans resulting in approximately 36,000 deaths in the United States annually (Wright and Webster, 2001; Prevention, 2005). These annual epidemics also have a large economic impact, and cause more than 100,000 hospitalizations per year in the United States alone. Influenza A viruses, which infect a wide number of avian and mammalian species, are responsible for the periodic widespread epidemics, or pandemics, that have caused high mortality rates (Wright and Webster, 2001). The most devastating pandemic occurred in 1918, which caused an estimated 20 to 40 million deaths worldwide (Reid et al., 2001). Less devastating pandemics occurred in 1957 and 1968. Influenza B virus infections comprise about 20% of the yearly cases, but influenza B virus, which appears to infect only humans, does not cause pandemics (Wright and Webster, 2001).
Influenza A and B viruses contain negative-stranded RNA genomes, which are in the form of eight RNA segments (Lamb and Krug, 2001). Most, but not all, of the corresponding genome RNA segments of influenza A and B viruses encode proteins of similar functions. Here we will focus on influenza A virus. The three largest genome segments encode the three subunits of the polymerase, PB1, PB2 and PA. The segment encoding PB1 also encodes a small nonstructural protein, PB1-F2, which has apoptotic functions. The middle-sized segments encode the hemagglutinin (HA), the nucleocapsid protein (NP) and the neuraminidase (NA). HA, the major surface protein of the virus, binds to sialic acid-containing receptors on host cells, and is the protein against which neutralizing antibodies are produced. NP protein molecules are bound at regular intervals along the entire length of each of the genomic RNAs to form ribonucleoproteins (RNPs), and also have essential functions in viral RNA replication. The NA viral surface protein removes sialic acid from glycoproteins. One of its major functions is to remove sialic acid during virus budding from the cell surface and from the HA and NA of the newly assembled virions, thereby obviating aggregation of the budding virions on the cell surface. The seventh genomic RNA segment encodes two proteins, M1 (matrix protein) and M2. The M1 protein underlies the viral lipid membrane, and is thought to interact with the genomic RNPs and with the inner (cytoplasmic) tails of the surface proteins, e.g., HA and NA. The M2 protein is an ion channel protein that is essential for the uncoating of the virus. The smallest segment encodes two proteins, NS1A and NS2. The NS2 protein mediates the export of newly synthesized viral RNPs from the nucleus to the cytoplasm. The NS1A protein is a multi-functional protein not that is not incorporated into virion particles (hence the designation “non structural”), and is discussed below.
The primary means for controlling influenza virus epidemics has been vaccination directed primarily against HA (Wright and Webster, 2001). However, the antigenic structure of the HA of influenza A virus can undergo two types of change (Wright and Webster, 2001). Antigenic drift results from the selection of mutant viruses that evade antibodies directed against the major antigenic type of the HA circulating in the human population. Mutant viruses are readily generated because the viral RNA polymerase has no proof-reading function. Because of antigenic drift, the vaccine has to be reformulated each year. Antigenic shift in HA results from reassortment of genomic RNA segments between human and avian influenza A virus strains, resulting in a new (potentially pandemic) virus encoding a novel avian-type HA that is immunologically distinct. The human population has little or no immunological protection against such a new virus. The viruses containing the H2 and H3 HA subtypes that caused pandemics in 1957 and 1968, respectively, resulted from the reassortment of avian and human genomic RNA segments (Wright and Webster, 2001). The HA of influenza B viruses undergoes antigenic drift, but not antigenic shift, because influenza B viruses do not have non-human hosts.
Pandemic influenza A viruses can also apparently arise by a different mechanism. It has been postulated that the 1918 H1 pandemic strain derived all eight genomic RNAs from an avian virus, and that this virus then underwent multiple mutations in the process of adapting to mammalian cells (Reid et al., 2004; Taubenberger et al., 2005). H5N1 viruses, which have already spread from Asia to Europe and Africa, appear to be undergoing this route for acquiring pandemic capability (Horimoto and Kawaoka, 2005; Noah and Krug, 2005). These viruses, which have been directly transmitted from chickens to humans, contain only avian genes, and are highly pathogenic in humans. The human mortality rate has been high, approximately 55% (WHO, 2006). H5N1 viruses have not yet acquired the ability for efficient transmission from humans to humans. Recent studies indicate that efficient human transmission will require more than the acquisition of the ability of HA to bind to human sialic acid receptors in the upper respiratory tract of mammalian organisms (Maines et al., 2006). However, at least one H5N1 gene, the PB2 gene, has already undergone adaptation to mammalian cells (Hatta et al., 2001). The vast majority of pathogenic H5N1 viruses have acquired a lysine at position 627 in the PB2 protein, in place of the glutamic acid that is found at this position in avian viruses. The presence of lysine at this position apparently enhances virus replication in mammalian cells, but the mechanism of enhancement has not been established (Crescenzo-Chaigne et al., 2002; Shinya et al., 2004).
Effective control of a H5N1 pandemic will require the use of antiviral drugs because it is not likely that sufficient amounts of an effective vaccine will be available, particularly in the early phase of a fast-spreading pandemic (Ferguson et al., 2005; Longini et al., 2005; Ferguson et al., 2006; Germann et al., 2006). Antivirals can be stockpiled, and if appropriately used, should limit the spread of pandemic influenza virus. The strategies that have been proposed for the use of antivirals to stem a H5N1 pandemic would also be expected to lead to more effective use of antivirals during annual influenza epidemics. Currently, there are two classes of antiviral drugs. One class, amantadine/rimantidine, is directed against the M2 ion channel of influenza A viruses (Pinto et al., 1992; Wang et al., 1993; Chizhmakov et al., 1996). Virus mutants resistant to this class of drugs rapidly emerge (Cox and Subbarao, 1999; Suzuki et al., 2003), and many of the human isolates of H5N1 viruses are already resistant to these drugs (Puthavathana et al., 2005). The other class of drugs is directed at NA, and is effective against both influenza A and B viruses (von Itzstein et al., 1993; Woods et al., 1993; Ryan et al., 1994; Gubareva et al., 1995; Kim et al., 1997; Mendel et al., 1998). However, H5N1 viruses that are partially, or completely, resistant to the NA inhibitor oseltamivir have been reported (de Jong et al., 2005; Le et al., 2005). The emergence of H5N1 viruses to these two classes of antiviral drugs highlights the need for additional antiviral drugs against influenza virus.